1. Field of the Invention
The present invention generally relates to braze furnaces for brazing of aluminum alloy structures in an oxygen-free atmosphere. More particularly, this invention relates to a convection braze furnace that is configured to include a duct system that directs and pulses a convective recirculated atmosphere through structures to be brazed, so as to create a multidirectional flow that achieves a more uniform temperature throughout the structures.
2. Description of the Prior Art
Heat exchangers are used in various capacities in automotive applications. For example, all automobiles having water cooled engines employ a radiator and a heater core. Automobiles equipped with air conditioning also include an evaporator and a condenser. These products are typically made from aluminum alloys and composed of two spaced header tanks interconnected by flow tubes having cooling fins extending therefrom. A cooling fluid is circulated through the header tanks and flow tubes, and air is directed over the heat exchanger so as to achieve the necessary temperature drop in the fluid.
The header tanks, flow tubes and cooling fins are attached to one another through a joining operation, most often a brazing operation in which the temperature of the assembled components is raised to melt a braze alloy that, upon cooling, rigidly joins the components to form a heat exchanger.
Although there are numerous ways to generate heat energy, heat transfer is basically limited to three modes; radiation, conduction and convection, which can be employed individually or in combination.
Many prior art braze furnaces rely on the exclusive use of radiant heat, whereby heat exchanger assemblies are transported through a ligated muffle tube that utilizes radiate heat energy to raise the temperatures of the assemblies to the braze melting temperature. This method for brazing has proven to be very popular among furnace manufacturers, for it has a very simple design and therefore is very practical to manufacture. Two designs have been primarily employed, one using natural gas burners ganged in a counter-firing manner and located in a cavity between an inner shell and an outer shell enclosing the inner shell. The other design uses electrical elements attached to the exterior of the inner shell. In both cases, the inner shell, or muffle, acts as a radiant surface from which the assemblies, as they lie inside the muffle, receive thermal energy. The outer shell, composed of a refractory material and a protective liner, serves as an insulating barrier to the surroundings.
While acceptable for some applications, radiant furnaces have inherent deficiencies as a result of nonuniform heat transfer to irregularly-shaped articles such as automotive heat exchanger assemblies, rendering such furnaces inefficient to the end user. In particular, assemblies constructed of components having various profile heights and mass densities, as is the case with heat exchangers, receive varying degrees of radiant energy as they lie in or pass through the muffle.
In brazing operations, it is important that all parts of a given assembly come to liquification temperatures at approximately the same time, so as to avoid excessive localized temperatures that can cause melting of the aluminum structure. With radiant braze furnaces, exposure time to the radiant energy source provides the only manner by which uniform assembly braze temperatures can be achieved. With this solution, heat energy, via conductance, is distributed over time. However, a significant disadvantage with this solution is that the cycle time is generally excessively long, necessitating the use of either a batch-type furnace or a continuous-type furnace requiring a large floor space. Such a consequence is unacceptable to most heat exchanger manufacturers for which productivity and manufacturing floor space are important concerns.
Under many circumstances, convection heat transfer offers a more effective and reliable means for achieving uniform temperatures in a given workpiece. Though assemblies having components with varying mass densities receive energy at differing rates, reduced temperature differences are realized in convection heat transfer than are possible by radiant heat transfer. Heat exchanger assemblies can be more readily and efficiently raised to the braze liquification temperature by employing the principles of convection heat transfer, whereby an impeller is used to circulate a suitable atmosphere within the brazing chamber and through the workpieces to be brazed.
Although convection furnaces offer significant advantages, the radiant furnaces have not been abandoned in their entirety because their muffle design offers a simple and effective means of maintaining an inert atmosphere as well as facilitating fabrication. As a result, furnace designs have been developed that incorporate both radiant and convection heat transfer principles. Examples of this type of design in the prior art are disclosed in U.S. Pat. No. 5,271,545 to Boswell et al., U.S. Pat. No. 4,501,387 to Hoyer, and U.S. Pat. No. 3,769,675 to Chartet. Furnace designs of the type represented by the above prior art have proven to be an improvement over radiation furnaces, and are widely used throughout the heat exchanger industry.
Attempts to design and build a furnace dependent solely on the principles of convection heat transfer have also been achieved. An example is the convection furnace taught by U.S. Pat. No. 5,195,673 to Irish et al. However, a shortcoming with the teachings of Irish et al. and many radiation-convection furnaces is that the heating atmosphere is directed through the workpiece, from top to bottom, such that nonuniform temperatures are created within the workpiece. A significant consequence is that the productivity of the equipment is limited to a single layer of parts to be brazed if uniform temperatures are to be achieved.
In contrast, the teachings of Chartet and U.S. Pat. No. 4,842,185 to Kudo et al. achieve a bidirectional flow through a heat exchanger by rotating the heat exchanger, thereby alternating the surfaces of the heat exchanger subjected to direct impingement by the heated atmosphere. Unfortunately, such teachings considerably complicate the construction of a braze furnace, adding cost to the furnace while significantly reducing throughput capacity.
Another method for achieving bidirectional heating of a heat exchanger is disclosed by Hoyer, which relies on a rotating heated atmosphere to flow upwardly through a first heat exchanger and then downwardly through a second and trailing heat exchanger. Unfortunately, uniform heating of the heat exchangers is difficult to achieve with this method in that more than two heat exchangers cannot be simultaneously heated in a uniform manner without considerably increasing the size of the furnace. Furthermore, the heat exchangers are directly subjected to radiant heat from heating elements located within the brazing chamber, which inherently causes localized heating at one surface of each heat exchanger. Finally, the braze chamber must be sufficiently large to accommodate radially-spaced stationary blades that are required to obtain the desired rotational flow of the heated atmosphere.
From the above, it can be seen that the full advantage for convection brazing has not been realized in the prior art. Furthermore, prior art attempts to braze solely by the convection method are only slightly more efficient than braze furnaces that incorporate the muffle/convection design. For example, during the brazing process, unidirectional, constant flow convection heat transfer methods tend to cause unnecessarily high localized temperatures within assemblies composed of components with differing densities. Another significant shortcoming is that the prior art does not make possible the circulation of a hot gas atmosphere within a braze furnace in a manner that enables the brazing of multiple layers of workpieces, i.e., workpieces stacked on a rack or conveyor. Therefore, although significant improvements have been achieved in obtaining uniform temperatures throughout a workpiece through the use of convection heat transfer methods, further improvements would be highly desirable.